1. Field of the Invention
The present invention relates to a bone anchor. More particularly, the present invention relates to a low profile bone anchor for fastening percutaneous medical devices to bone.
2. Description of the Prior Art
In certain surgical procedures and medical examination techniques, it is necessary for points or sites to be determined and identified within the body. For instance, stereotactic surgery requires a probe or electrode to be advanced into a patient's brain via a small aperture to a deep-seated brain path or nerve nucleus, while preserving adjacent structures. Likewise, certain imaging medical examination medical procedures, such as Positron Emission Tomography (PET), Roentgen Ray Analysis (X-ray), Computed Tomography (CT), and Nuclear Spin Resonance Tomography (MMR) require identification and location of inner body structures with a high degree of precision.
Conventionally, inner-body locations are pinpointed by percutaneously implanting external fixation pins into the patient's bone, such as his/her skull, to allow attachment of markers or various adaptors or other devices. Each marker contains a substance which provides a contrasting image and the plurality of markers, generally three, can effectively be triangulated to pinpoint inner-body locations. Moreover, the markers can be maintained in position which allows the markers to be positioned in advance of the surgical or examination procedure as well as providing repeatability for follow-on or additional treatments.
Typically, markers, often referred to as fiducial markers, are attached to the body through anchors drilled into bone, such as the skull, in a plurality of locations. The marker can be integral with the anchor, for instance the anchor can be formed of a marker (high contrast) material or, contrariwise, anchors can be installed in the body and then a marker attached to each anchor.
When the anchor system is used, it is possible to attach more than simply a marker to the anchor. For instance, certain devices needed for performing stereotactic surgery can be positioned at the skull using a bone anchor, such as sterotactic frames used to replace older “halo” type devices. Thus, the surgical device is repeatedly and precisely positioned for the procedure.
Conventional bone anchor devices have several drawbacks. First of all, the anchor often protrudes from the skull or other body part to such a degree as to run the risk of snagging or being caught on objects, thus creating the risk of breaking, infection, and causing the anchor to move out of position, in addition to the simple embarrassment and physical discomfort of a protruding anchor. Moreover, attachment methods conventionally used are not as precise as would be desired. Indeed, when a plurality, such as three, anchors is employed, even slight imprecision in each anchor is magnified and exacerbated when the imprecision occurs at each of three anchors.
In one instance of a device suggested for use as a fiducial marker, Leibinger, Leiginger, Felber, and Plangger, in U.S. Pat. No. 5,394,457, disclose a device for marking body sites for medical examinations. The described device consists of an anchor portion slip-fit into bone and a marker then slip-fit into the anchor portion. Clearly, the slip-fitting of the anchor into the bone and the marker into the anchor provide two areas where the fit may not be as precise as may be desired, creating imprecision in location. Although, Leibinger et al., disclose an embodiment where a screw can be inserted through the bottom of the anchor to provide a different anchoring mechanism, there is still a great deal of imprecision inherent in what would then be a three-part system (screw, anchor, marker).
In U.S. Pat. No. 5,397,329, Allen discloses a fiducial implant for a human body. The implant consists of an anchoring portion, which can be threaded, and a marker portion which extends above the bone to which the anchor portion is anchored.
As can be seen, neither of the discussed patents provides the flexibility of an anchor system to which a marker or other device can be removably attached, yet with the degree of precision and low profile sought.
What is desired therefore, is a bone anchor system which provides a high degree of precision in locating points in the center of a body, such as the brain, yet which allows either attachment of a marker or other surgical devices, while still maintaining a low profile with respect to the patient's body.